I'm not really sure what this error means since I'm building with python 2.7, I don't have -python_single_target_python2_7 anywhere and scipy claims it can be built against python 2.7 (the any-of ..... line).

My guess is that the unstable scipy-0.12.0 already uses the new Python-r1 eclass, while stable cython-0.16 doesn't, which makes it impossible to satisfy the python* flags.

My recommendation is to update to a newer cython package that uses the python-r1 eclass.

I can support this claim by:

Code:

gentbox ~ # emerge -va scipy

These are the packages that would be merged, in order:

Calculating dependencies... done!

The following keyword changes are necessary to proceed:
(see "package.accept_keywords" in the portage(5) man page for more details)
# required by sci-libs/scipy-0.12.0
# required by scipy (argument)
=dev-python/cython-0.19.1 ~amd64

Would you like to add these changes to your config files? [Yes/No] n

emerge: there are no ebuilds built with USE flags to satisfy "virtual/python-imaging[python_targets_python2_5(-)?,python_targets_python2_6(-)?,python_targets_python2_7(-)?,python_targets_python3_2(-)?,python_targets_python3_3(-)?,-python_single_target_python2_5(-),-python_single_target_python2_6(-),-python_single_target_python2_7(-),-python_single_target_python3_2(-),-python_single_target_python3_3(-)]".
!!! One of the following packages is required to complete your request:
- sci-libs/scipy-0.12.0::gentoo (Change USE: -python_targets_python3_2 -python_targets_python2_7, this change violates use flag constraints defined by sci-libs/scipy-0.12.0: 'any-of ( python_targets_python2_5 python_targets_python2_6 python_targets_python2_7 python_targets_python3_2 python_targets_python3_3 )')EMERGE_DEFAULT_OPTS="--autounmask=n"
(dependency required by "sci-libs/scipy-0.12.0" [ebuild])
(dependency required by "scipy" [argument])

So as you can see, autounmask has the same recommendation.

You give yourself an unnecessary headache by having

Code:

EMERGE_DEFAULT_OPTS="--autounmask=n"

I trust autounmask these days and when using --ask, it's also convenient to use

Code:

EMERGE_DEFAULT_OPTS="--autounmask-write"

It asks if you want to make the changes and then you can selectively apply them with dispatch conf.